In one type of shadow mask cathode ray tube, a mosaic phosphor screen is provided as an array of phosphor lines which extend from the top to the bottom of the screen. A shadow mask electrode is disposed adjacent to the screen and comprises a formed metal sheet having an array of elongated slot-type apertures therein. The apertures are disposed in rows which are aligned with the phosphor lines and are elongated in the direction of the rows. Each of the apertures is separated from an adjacent aperture in the same row by a web.
In fabricating this type of shadow mask, the desired array of apertures is first etched in a flat metal sheet and then the metal sheet is formed to the desired shape. The forming may, e.g. be accomplished in a die and mold operation to produce an article usually with a curved, e.g. domed, self-supporting central region having a bent over cylindrical skirt at the periphery thereof. The forming operation necessarily involves a stretching of the apertured sheet transversely of the aperture rows, which often times causes a rupturing of the webs between the apertures. This stretching and web-rupturing is especially present near the periphery of the mask where the mask may be provided with a sharp bend to produce the cylindrical skirt region which telescopes within or over (as shown in FIG. 1) a frame member. The reason for this rupturing is that the sheet material between adjacent rows of apertures is much more massive, and hence stronger, than the webs themselves. Thus, in a stretching operation, almost all of the stretch occurs in the webs.
The stretch and rupture problem of forming slot-type apertured masks has been recognized by the prior art. The patentees in U.S. Patent 3,916,243 discuss this problem and therein propose the addition of elongated channels extending from the top of the mask to the bottom of the mask between the adjacent rows of apertures in order to provide relief of rupture-producing stress in the webs during forming of the mask. However, the inclusion of such channels creates a new problem of excessive weakening of the formed mask against denting caused by accidental rough handling and against doming caused by thermal expansion of the mask during operation in the finished tube. Furthermore, use of straight channels does not allow optimum positioning of the stress relief means.